In recent years, along with the spread of mobile electronic devices such as mobile phones and notebook-sized personal computers, development of smaller and lighter nonaqueous electrolyte secondary batteries having a high energy density has been strongly demanded.
Development of high-power secondary batteries as batteries for electric automobiles typified by hybrid automobiles has also been strongly demanded.
The secondary batteries that meet such demands are exemplified by lithium ion secondary batteries. Lithium ion secondary batteries include a negative electrode, a positive electrode, an electrolytic solution and the like, in which a material into and from which lithium can be inserted and desorbed has been used as an active material for the negative and positive electrodes.
Research and development of the lithium ion secondary batteries have been extensively carried out at present, and in particular, lithium ion secondary batteries in which a layer or spinel type lithium metal composite oxide is used as a positive electrode material can give a voltage as high as 4 V; therefore, practical applications thereof as batteries having a high energy density have been accelerated.
As positive electrode materials for use in such lithium ion secondary batteries, lithium cobalt composite oxide (LiCoO2) that can be relatively easily synthesized, lithium nickel composite oxide (LiNiO2) in which nickel less expensive than cobalt is used, lithium nickel cobalt manganese composite oxide (LiNi1/3Co1/3O2), lithium manganese composite oxide (LiMn2O4) in which manganese is used, and the like have been hitherto proposed. Among these, lithium nickel cobalt manganese composite oxide has been receiving attention as a material in which excellent cycle characteristics can be achieved and high power can be obtained with a low resistance, when being used as a positive electrode.
As requirements for achieving the above favorable performances of positive electrodes (excellent cycle characteristics, low resistance and high power), positive electrode materials are required to include particles having a uniform and appropriate particle diameter.
The grounds for such requirements are that use of a material having a large particle diameter and a small specific surface area leads to failure in reserving a sufficient area for reaction with the electrolytic solution, thereby resulting in an increase of the reaction resistance and failure in obtaining a battery having high power, and that use of a material having a broad particle size distribution leads to reduction of the battery capacity, thereby resulting in defects such as an increase of the reaction resistance. The battery capacity is reduced because the voltage applied to the particles in the electrode becomes ununiform, so that fine particles selectively deteriorate due to repetition of charge and discharge.
Therefore, it is necessary to produce particles so as to have an appropriate and uniform particle diameter also in the case of the aforementioned lithium nickel cobalt manganese composite oxide in order to improve performances of the positive electrode material.
Since lithium nickel cobalt manganese composite oxides are generally produced from a composite hydroxide, it is necessary to use one having a small and uniform particle diameter as a composite hydroxide employed as a raw material thereof in order to obtain particles having an appropriate and uniform particle diameter.
That is, for improving performances of the positive electrode material to produce a final product, or a lithium ion secondary battery having high performances, it is necessary to use a composite hydroxide including particles having a small particle diameter and a narrow particle size distribution, as the composite hydroxide employed as a raw material of the lithium nickel cobalt manganese composite oxide for forming the positive electrode material.
Concerning a method for producing a composite hydroxide, various proposals have been made up to the present (Patent Literatures 1 to 3).
Patent Literature 1, for example, discloses that a nickel cobalt manganese composite hydroxide is precipitated by continuously or intermittently applying an aqueous solution of nickel-cobalt-manganese salt, an aqueous solution of an alkaline metal hydroxide and an ammonium ion donor to a reaction system, adjusting a temperature of the reaction system to an almost constant value within a range of 30° C. to 70° C., and making the reaction proceed with its pH maintaining at an almost constant value within a range of 10 to 13. Patent Literature 1 also discloses that an intermediate having a preferable particle size distribution can be obtained by a multi-stage reaction than a one-stage reaction, and a part of produced particles may be returned to a reaction vessel for controlling a property of the produced particles.
Patent Literature 2 discloses a method for producing a cathode active material for a lithium secondary battery, in which lithium-coprecipitated composite metal salt with its particles having a approximately spherical shape is synthesized by continuously supplying to a reaction vessel an aqueous solution of composite metal salt with the salt concentration adjusted by dissolving the salt of each structural element of the substance described above in water, a water soluble complexing agent that forms metal ions and complex salt, and an aqueous lithium hydroxide solution and generating composite metal complex salt; then decomposing the complex salt by the lithium hydroxide to cause the lithium-coprecipitated composite metal salt to be precipitated; and repeating the generation and decomposition of the complex salt while circulating in the vessel to obtain the lithium-coprecipitated composite metal salt by overflowing the same. The cathode active material obtained by this method using composite metal salt as a raw material reportedly has a high filling density, a uniform composition and a nearly spherical shape.
Patent Literature 3 proposes a method for producing a cathode active material for a nonaqueous electrolyte batteries, in which an oxide or a hydroxide as a precursor is obtained by simultaneously introducing into a reaction vessel an alkali solution with an aqueous solution containing at least two kinds of transition metal salt or at least two kinds of aqueous solutions of each different transition metal salt, and then performing coprecipitation while allowing a reducing agent to coexist or an inert gas to flow. This method itself is not for controlling a particle diameter but for preventing an imperfect solid solubility at an atomic level. An apparatus for obtaining a hydroxide or an oxide having a spherical shape, a high density and a large particle diameter, however, is disclosed therein.
The apparatus employs a system described below. A mixture of an aqueous solution is caused to flow from bottom to top, so that crystal particles whose specific gravity increases because of growth of their crystals to some extent settle out to reach a collecting portion at the bottom. On the other hand, ungrown crystal particles do not reach the bottom because they are pushed back by a force of the solution flowing from the bottom. That is, the apparatus is for obtaining crystal particles having a large particle diameter by classifying and collecting the generated crystals.
However, Patent Literature 1 does not disclose a specific method for controlling the particle size distribution or a property of the produced particles, but merely discloses that the composite hydroxide was obtained at a constant temperature and pH in Examples.
Patent Literature 2 discloses a continuous crystallization method in which the product is obtained by overflowing. The particle size distribution is therefore likely to be spread to provide a normal distribution, and thus to obtain particles having an almost uniform particle diameter is difficult.
Further, Patent Literature 3 discloses the technique for obtaining crystal particles having a large particle diameter by classifying and obtaining generated crystals. It is however considered that the production conditions need to be strictly controlled in order to obtain a product having a uniform particle diameter, leading to difficulty in production in an industrial scale.
As described above, although various methods for producing a composite hydroxide have been studied, a method capable of producing, in an industrial scale, a composite hydroxide having a small particle diameter with high uniformity thereof has not been developed at present. In order to improve performances of lithium secondary batteries, a method for producing such a composite hydroxide has been demanded.